1. Field of the Invention
The present invention relates to a micro-oscillation element that has an oscillating section for which rotary displacement is possible. More particularly, the present invention relates to a micro-oscillation element which can be applied to a micro-mirror element, which is built into such optical devices as optical measurement devices for performing precision measurement using light waves, optical disk devices for recording and reproducing data for optical disks, and optical switching devices for switching the optical paths between a plurality of optical fibers, so as to change the traveling direction of the light.
2. Description of the Related Art
Recently the application of MEMS (Micro-Electro-Mechanical-System) devices, which have a microstructure created by micro-machining technology, are being attempted in various technical fields. For example, in the field of optical communication technology, micro-mirror elements having light reflecting functions are receiving attention.
A micro-mirror element has a mirror face for reflecting light, and can change the reflecting direction of the light by the oscillation of this mirror face. Electrostatic driving type micro-mirror elements, which use electrostatic force for oscillating the mirror face, are used for many optical devices. Electrostatic drive type micro-mirror elements can roughly be classified into two: micro-mirror elements manufactured by so called surface micro-machining technology, and micro-mirror elements manufactured by so called bulk micro-machining technology.
In surface micro-machining technology, material thin films corresponding to each composing region are processed into a desired pattern on the substrate, and by layering such patterns sequentially, each region of constructing elements, such as a support, mirror face and electrode, and a sacrifice layer to be the removed layer, are created. An electrostatic driving type micro-mirror element by such a surface micro-machining technology is disclosed, for example, in Japanese Patent Application Laid-Open No. H7-287177. In bulk micro-machining technology, on the other hand, the support and mirror section are formed into a desired shape by etching the material substrate itself, and the mirror face and electrode are created with a thin film when necessary. A bulk micro-machining technology is disclosed, for example, in Japanese Patent Application Laid-Open No. H9-146032, No. H9-146034, No. H10-62709 and No. 2001-13443.
One technical issue which is required for micro-mirror element is the high flatness of the mirror face which reflects light. However, according to surface micro-machining technology, where a mirror face to be finally created is thin, the mirror face tends to warp, and it is difficult to implement high flatness on the entire mirror face over a wide area. Whereas in bulk micro-machining technology, where a mirror face is disposed on the mirror section which is created by etching a relatively thick material substrate itself using etching technology, the rigidity of the mirror section can be more easily insured over a wide area of the mirror face. Therefore bulk micro-machining technology is preferable for creating a mirror face which has a sufficiently high optical flatness.
FIG. 24 and FIG. 25 show a conventional electrostatic driving type micro-mirror element X6 fabricated by bulk micro-machining technology. FIG. 24 is an exploded perspective view of the micro-mirror element X6, and FIG. 25 is a cross-sectional view along the XXV-XXV line in FIG. 24, of an assembled micro-mirror element X6.
The micro-mirror element X6 has a structure where the mirror substrate 60 and the base substrate 66 are layered. The mirror substrate 60 is comprised of a mirror section 61, a frame 62 and a pair of torsion bars 63 connected to these. The external forms of the mirror section 61, frame 62 and the pair of torsion bars 63 can be formed by etching one side of a predetermined material substrate, such as a silicon substrate which has conductivity. On the front face of the mirror section 61, the mirror face 64 is created. On the rear face of the mirror section 61, a pair of electrodes 65a and 65b are disposed. The pair of torsion bars 63 specifies the rotation axis A6 in the later mentioned oscillation operation of the mirror section 61. On the base substrate 66, the electrode 67a, which faces the electrode 65a of the mirror section 61, and the electrode 67b, which faces the electrode 65b thereof, are disposed.
In the micro-mirror element X6, when potential is applied to the frame 62 of the mirror substrate 60, the potential is transferred to the electrode 65a and the electrode 65b via the pair of torsion bars 63 and the mirror section 61, which are integrated with the frame 62 using a same conductive material. Therefore by applying a predetermined potential to the frame 62, the electrodes 65a and 65b can be charged to positive, for example. If the electrode 67a of the base substrate 66 is charged to negative in this status, electrostatic attraction is generated between the electrode 65a and the electrode 67a, and the mirror section 61 oscillates in the direction of the arrow M6 with twisting the pair of torsion bars 63, as shown in FIG. 25. The mirror section 61 can oscillate to the angle at which the electrostatic attraction between the electrodes and the total of the torsional resistance of the torsion bars 63 balance. If the electrode 67b is charged to negative in the status where the electrodes 65a and 65b of the mirror section 61 are charged to positive, on the other hand, electrostatic attraction is generated between the electrode 65b and the electrode 67b, and the mirror section 61 can oscillate to a direction opposite of the arrow M6. By such an oscillation driving of the mirror section 61, the reflection direction of the light, which is reflected by the mirror face 64, can be switched.
In order to increase the resonance frequency of the mirror section 61 (e.g. several hundred kHz or more), to meet the demand for high-speed operation in the micro-mirror element X6, a method of increasing the rigidity of the torsional direction (torsional rigidity) of the torsion bar 63 to increase the rotational rigidity of the mirror section 61, or a method of decreasing the mass of the mirror section 61 so as to decrease the moment of inertia thereof (polar moment of inertia of area with respect to the axis A6) may be used in prior art. Generally the resonance frequency of the mirror section (region which performs the rotary operation) of the element is given by the following formula (1). In formula (1), f0 is a resonance frequency of the mirror section, K is a rotational rigidity of the mirror section, and I is a moment of inertia of the mirror section (polar moment of inertia of area with respect to the axis).
                              f          0                =                              K            I                                              (        1        )            
According to formula (1), for the mirror section 61 of the micro-mirror element X6, a higher resonance frequency can be obtained as the rotational rigidity thereof is higher, or as the moment of inertia thereof is lower. As the torsional rigidity of the torsion bar 63 is higher, the rotational rigidity of the mirror section 61 becomes higher, and the potential energy to be stored in this torsion bar 63, when the mirror section 61 oscillates in one direction, increases, and therefore the quantity of potential energy, which is released from the torsion bar 63 when the mirror section 61 oscillates to the opposite direction after this oscillation in one direction, and is converted into kinetic energy, increases, which is suitable for obtaining a high resonance frequency. As the moment of inertia of the mirror section 61 decreases, the driving force required for a predetermined oscillation operation of the mirror section 61 is smaller, so this is suitable for obtaining a high resonance frequency.
A known method of increasing the torsional rigidity of the torsion bar 63 is increasing the thickness and width of the torsion bar 63 so as to increase the cross-section area thereof. However, the thickness of the torsion bar 63 must be set to the thickness of the mirror section 61 or less, for practical reasons. If the thickness of the torsion bar 63 exceeds the thickness of the mirror section 61, then the torsional rigidity of the torsion bar 63 increases excessively, and in this case even if a driving force of the mirror section 61 is generated, this torsion bar 63 cannot be appropriately twisted, and the mirror section 61 tends to deform. If the width of the torsion bar 63 is inappropriately increased, this too increases excessively the torsional rigidity of the torsion bar 63, and in this case, even if a drive forcing of the mirror section 61 is generated, this torsion bar 63 cannot be appropriately twisted, and the mirror section 61 tends to deform.
A known method for decreasing the moment of inertia of the mirror section 61 by decreasing the weight thereof is decreasing the thickness of the mirror section 61. However, as the mirror section 61 becomes thinner, the mirror section 61 itself tends to deflect, and assuring the flatness of the mirror face 64 tends to become difficult. If the flatness of the mirror face 64 cannot be sufficiently assured, an appropriate light reflection function cannot be implemented in the micro-mirror element X6.
In this way, prior art has difficulty to operate a micro-mirror element or a micro-oscillation element at a high resonance frequency.